9.1 Net Ionic Equations and
Qualitative Analysis
Learning Goals …
… write an ionic and net ionic equation
… use qualitative analysis (flame test, solution
colour and solubility) to identify ions in a
solution
Let’s take a closer look at the reaction between
NaCl (aq) + AgNO3 (aq) !
NaNO3 (aq) + AgCl (s)
What actually happened?
Cl- (aq) + Ag+ (aq) ! AgCl (s)
SPECTATOR
IONS
The Na+ and NO3- are still in the solution
A chemical equation written without the spectator ions is
called a net ionic equation. A net ionic equation includes
only those ions or compounds that undergo chemical
change.
TYPES OF EQUATIONS
i.
Balanced Chemical equation
ii.
Ionic equation
iii.
Net ionic equation
Zinc
+
chloride
zinc
sodium
!
phosphate
phosphate
i.3ZnCl2 (aq) +
6
Na
2 3PO4(aq)!
Zn3(PO4)2 (s) +
ii. 3Zn+2(aq) + 6Cl-(aq) + 6Na+(aq) +
2PO4-3(aq)
iii.
+
sodium
chloride
3Zn+2(aq) + 2PO4-3(aq)!
Spectator ions: Na+ and Cl-
Zn3(PO4)2(s)
!
NaCl(aq)
Zn3(PO4)2(s) +
6Na+(aq) + 6Cl(aq)
Now you try one …
Lead (II) + sodium
nitrate
sulfide
i.
Pb(NO3)2 (aq) +
!
Lead (II) + sodium
sulfide
nitrate
Na2S(aq) !
PbS(s) + 2 NaNO3(aq)
ii. Pb+2(aq) + 2 NO3-(aq) + 2Na+(aq) + S-2(aq) !
PbS (s)
+ 2Na+ + 2NO (aq)
3 (aq)
iii. Pb+2(aq) + S-2(aq)!
Spectator ions:
PbS(s)
Na+ and NO3-
Qualitative Analysis
• Qualitative Analysis identifies a substance in a
sample by observation of physical and chemical
properties.
• You can often identify whether a cation ion is in a
sample by observing one or more of the following:
• Flame Test results
• solution colour
• precipitates formed with select aqueous
solutions.
• Qualitative analysis can tell you what ions are present
1. Flame Test
Uses the colour that a sample produces in a flame to
identify the metal ion in the sample
Flame Colours of Some Metals
Ion
Li+
Na+
K+
Cs+
Colour
Crimson Red
Yellow-orange
Purple
Blue
Ca2+
Sr2+
Cu2+
Pb2+
Red-orange
Bright red
Bluish-green
Bluish-white
Fast forward video file to 25 seconds in…
2. Colours of Ions in Solution
Uses the colour of a solution to identify the metal ion
in the sample
Ion
Cr2+
Cu2+
Cr3+
Cu+
Fe2+
Ni2+
Fe3+
Co2+
Mn2+
Colour
blue
Pb2+
Bluish-white
green
Pale yellow
Pink
3. Precipitate Formation
Uses the solubility properties of ions to identify an unknown
ion. We can add a known reactant to a solution and observe
whether a precipitate forms.
Let’s say we wanted to precipitate Pb2+ out of a water
sample…
Let’s look at our Solubility Table …
Lead (II) ions can be precipitated with sulfate ions to yield
solid lead (II) sulfate.
Pb2+ + SO42- ! PbSO4 (s)
We can filter off the precipitate and separate it out of the
solution.
SO42- could be obtained from a solution of Na2SO4 (we would
need a nonreactive cation)
Ex) An ion in solution forms a yellow precipitate when
sodium iodide, NaI (aq), is added to the solution. The
precipitate produces a blue-white colour when it is
heated in a flame. What metal
ion might be present
in the solution?
According to the Solubility Rules,
iodides are soluble except if it contains Pb2+, Ag+, and
Hg+ and Hg22+. So the ion must be one of these four.
We can then look to the flame test results to narrow the
ion down.
Pb2+ has a blue-white flame so this must be the ion
present.
CAN I …
… write an ionic and net ionic equation
… use qualitative analysis (flame test and
solubility) to identify ions in a solution
HOMEWORK
p410 #5-10
p414 #12-14
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Flame Colours of Some Metals
Ion
Li+
Na+
K+
Cs+
Colour
Crimson Red
Yellow-orange
Purple
Blue
Ca2+
Sr2+
Cu2+
Pb2+
Red-orange
Bright red
Bluish-green
Bluish-white
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